[PATCH] ring-buffer: Add writer_loops to stat file

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

As the ring buffer never had a cmpxchg loop until the reserving logic
added one, add a statistic on how many times it happens. This can be used
to make sure there's not any issues because the writer is retrying.

 ~# cat /sys/kernel/tracing/per_cpu/cpu0/stats
entries: 51226
overrun: 3920945
commit overrun: 0
bytes: 1441376
oldest event ts:  2881.254827
now ts:  2912.011953
dropped events: 0
read events: 0
writer loops: 267

Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
[ Depends on:
  https://lore.kernel.org/linux-trace-kernel/20240118181206.4977da2f@xxxxxxxxxxxxxxxxxx/
]

 include/linux/ring_buffer.h |  1 +
 kernel/trace/ring_buffer.c  | 22 +++++++++++++++++++++-
 kernel/trace/trace.c        |  3 +++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index fa802db216f9..f2bd59f24f7a 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -180,6 +180,7 @@ unsigned long ring_buffer_overrun_cpu(struct trace_buffer *buffer, int cpu);
 unsigned long ring_buffer_commit_overrun_cpu(struct trace_buffer *buffer, int cpu);
 unsigned long ring_buffer_dropped_events_cpu(struct trace_buffer *buffer, int cpu);
 unsigned long ring_buffer_read_events_cpu(struct trace_buffer *buffer, int cpu);
+unsigned long ring_buffer_writer_loops(struct trace_buffer *buffer, int cpu);
 
 u64 ring_buffer_time_stamp(struct trace_buffer *buffer);
 void ring_buffer_normalize_time_stamp(struct trace_buffer *buffer,
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 74f8895785ca..a8a32c15c8b6 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -474,6 +474,7 @@ struct ring_buffer_per_cpu {
 	local_t				pages_touched;
 	local_t				pages_lost;
 	local_t				pages_read;
+	local_t				writer_loops;
 	long				last_pages_touch;
 	size_t				shortest_full;
 	unsigned long			read;
@@ -3483,8 +3484,10 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 
  /*B*/	rb_time_set(&cpu_buffer->before_stamp, info->ts);
 
- /*C*/	if (!local_try_cmpxchg(&tail_page->write, &w, w + info->length))
+ /*C*/	if (!local_try_cmpxchg(&tail_page->write, &w, w + info->length)) {
+		local_inc(&cpu_buffer->writer_loops);
 		goto again;
+	}
 
 	/* Set write to the start of this event */
 	write = w & RB_WRITE_MASK;
@@ -4062,6 +4065,22 @@ u64 ring_buffer_oldest_event_ts(struct trace_buffer *buffer, int cpu)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_oldest_event_ts);
 
+/**
+ * ring_buffer_writer_loops - get the number of times writer failed cmpxchg
+ * @buffer: The ring buffer
+ * @cpu: The per CPU buffer to read from.
+ */
+unsigned long ring_buffer_writer_loops(struct trace_buffer *buffer, int cpu)
+{
+	struct ring_buffer_per_cpu *cpu_buffer;
+
+	if (!cpumask_test_cpu(cpu, buffer->cpumask))
+		return 0;
+
+	cpu_buffer = buffer->buffers[cpu];
+	return local_read(&cpu_buffer->writer_loops);
+}
+
 /**
  * ring_buffer_bytes_cpu - get the number of bytes unconsumed in a cpu buffer
  * @buffer: The ring buffer
@@ -5145,6 +5164,7 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
 	local_set(&cpu_buffer->pages_touched, 0);
 	local_set(&cpu_buffer->pages_lost, 0);
 	local_set(&cpu_buffer->pages_read, 0);
+	local_set(&cpu_buffer->writer_loops, 0);
 	cpu_buffer->last_pages_touch = 0;
 	cpu_buffer->shortest_full = 0;
 	cpu_buffer->read = 0;
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 46dbe22121e9..3a5f33c344c4 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8670,6 +8670,9 @@ tracing_stats_read(struct file *filp, char __user *ubuf,
 	cnt = ring_buffer_read_events_cpu(trace_buf->buffer, cpu);
 	trace_seq_printf(s, "read events: %ld\n", cnt);
 
+	cnt = ring_buffer_writer_loops(trace_buf->buffer, cpu);
+	trace_seq_printf(s, "writer loops: %ld\n", cnt);
+
 	count = simple_read_from_buffer(ubuf, count, ppos,
 					s->buffer, trace_seq_used(s));
 
-- 
2.43.0